1. Field of the Invention
The present invention relates to a laminated piezoelectric element, an actuator and a printing head and, more particularly, to a laminated piezoelectric element and an actuator that are suitable for the piezoelectric sensor of, for example, a fuel injector, an ink jet printer, a piezoelectric resonator, an oscillator, an ultrasonic motor, an ultrasonic oscillator, a filter, an acceleration sensor, a knocking sensor, an AE sensor or the like, and can be advantageously used particularly for a printing head that utilizes vibration of a 2-dimensional vibration of expansion and contraction or linear vibration in the longitudinal direction or in the direction of the thickness.
2. Description of Related Art
Piezoelectric ceramics materials have been used in, for example, an actuator, a filter, a piezoelectric resonator (oscillator included), an ultrasonic oscillator, an ultrasonic motor and a piezoelectric sensor.
Among these devices, the piezoelectric actuator is used as the positioning actuator for an X-Y stage of semiconductor manufacturing equipment, the actuator for the printing head of an ink jet printer or the like, by making advantage of the very high response of the piezoelectric element to electrical signals, in the order of micro seconds. Especially with the recent trend of color printers toward higher printing speed and lower prices, piezoelectric elements are under increasing demand for the application to the ink discharging actuator of the ink jet printer or the like.
An actuator that uses a silver-palladium alloy for internal electrode is conventionally known. This actuator is manufactured in the following process. First, an electrically conductive paste is printed on the surface of a green sheet made of a piezoelectric ceramic material as the major component with a thickness of 200 μm so as to form internal electrodes. Second, 200 green sheets are stacked one on another with the side on which the internal electrodes are printed facing upward. A set of five green sheets without the electrode paste printed thereon is then stacked on each side of the stack, on top and bottom, and the resultant stack is pressed to make a laminate. Next, the laminate is processed to remove binder contained in the green sheets and in the internal electrodes, and is sintered to make a sintered laminate. Further, an insulator, external electrodes and lead wires are connected to the sintered laminate, thereby to complete the actuator.
The actuator that is made as described above has an advantage that it is easy to make the multi-layer laminate of the piezoelectric ceramic material and the electrode material and that the actuator can be manufactured at a low cost, and therefore has been preferably used as the actuator for the printing head of ink jet printers, the positioning actuator for the X-Y stage and the like.
Unfortunately, the conventional actuator is relatively large in thickness which imposes a limitation to the amount of displacement, resulting in a problem that large displacement cannot be achieved. Moreover, there is also a problem of deteriorating characteristics of the actuator displacement, thus resulting in a marked decrease in the displacement of the printing head.
In high resolution printers which have been showing remarkable progress recently, in particular, thinner piezoelectric ceramic layers are employed in order to achieve greater displacement of the actuator. In the case of a thin actuator made by stacking piezoelectric ceramic layers each measuring several tens of micrometers or less in thickness or an actuator having a total thickness of 100 μm or less, on the other hand, there is such a problem that shrinkage of the internal electrodes caused during sintering results in significant deformation because the actuator is very thin.
There is also such a problem that the d constant varies significantly across a single element, since residual stress is distributed unevenly across the actuator by uneven shrinkage of the internal electrodes. In such a thin actuator as described above, in particular, in case a plurality of displacement elements are mounted on a single circuit board, significant variations beyond ±10% occur in the amount of displacement among the actuators. It requires an expensive IC to control the operation of the actuators having such significant variations, thus resulting in an increase in the manufacturing cost of the printing head or the printer, while requiring complicated control scheme.